This invention concerns methods and apparatus for grinding workpieces which are composed of concentric in-line cylindrical regions and intermediate non-axial non-circular or eccentric regions. Examples of such workpieces are camshafts and crankshafts of internal combustion engines and such workpieces are referred to herein as composite workpieces.
Because of the different techniques used for grinding in-line and off-axis regions of workpieces, it has hitherto been commonplace to grind the cylindrical region of a composite workpiece on one grinding machine and to transfer the workpiece to another grinding machine for grinding the non-axial regions such as cam lobes or crankpins.
With the trend towards lightweight engine components, camshafts and crankshafts have become less stiff and more prone to distortion as a result of grinding forces exerted on the workpiece by the grinding wheel particularly when high metal removal rates are desired. To this end it has been proposed to resist grinding forces exerted by the grinding wheel by means of so-called worksteadies or workrests which engage diametrically opposite regions of the workpiece without inhibiting rotation, to resist the bending moment created by the grinding wheel forces exerted on the workpiece.
In general the workrests have been applied against the journal bearing regions of the workpieces, i.e., the cylindrical co-axial regions of the workpiece which are normally intermediate non-circular or off-axis components, such as the cam lobes and crankpins of the exemplary workpieces.
It is an object of the present invention to provide a single machine for grinding composite workpieces.
It is a further object of the invention to improve the rigidity of the mounting for a workrest as incorporated in such a machine.
According to one aspect of the present invention in a grinding machine comprising a stationary support structure, a wheelhead assembly slidable relative to the said structure in a direction perpendicular to a workpiece axis, headstock and footstock means mounted on the structure and defining the workpiece axis, rotating a workpiece mounted therebetween, at least one workrest slidably adjustable along at least one rigid elongate member or rail which extends generally parallel to the workpiece axis, and programmable computer based control means for controlling the movement of the wheelhead, the rotation of the workpiece, and engagement and disengagement of the workrest with a cylindrical region of the workpiece, wherein means is provided for fixing the workrest at a specific axial location along the length of the said elongate rail so that the workrest aligns with a cylindrical region of the workpiece.
During grinding, swarf, coolant and grinding medium particles will be present in the environment around the interface between the grinding wheel and the workpiece, and in order to prevent any such material from reaching the sliding surface of the elongate rail on which the workrest slides, and for axially positioning the workrest along the rail, cover means is provided on opposite sides of the workrest to keep any such unwanted material away from the surface of the elongate support rail therebelow, and prevent lateral movement of the workrest from its selected position.
The workrest and the cover means preferably form a linear bearing with the rail.
Preferably the rail is made up of two spaced apart parallel rails.
According to a preferred feature of the invention, each of the covers forming the cover means is rigid and structural and either adjustable in length or available in different lengths to enable differently sized gaps measured parallel to the axis of the workpiece to be covered by the covers, depending on the desired position of the workrest.
The covers may be clamped in an axial sense so as to clamp between them the workrest and position the latter along the workpiece axis.
Where the sealing between the workrest and the cover means is insufficiently reliable to prevent the Penetration of fine particle material and fluid, telescoping covers may be provided below the rigid cover means which are sealed at least to the opposite sides of the workrest and either extend sufficiently far axially along the length of the elongate rail to prevent the ingress of unwanted particulate or fluid material, or are sealed at their opposite ends to end faces of support members between which the elongate rail extends, thereby forming a sealed enclosure within which the elongate rail is protected. The telescoping nature of the inner covers enables the workrest to be moved axially for adjusting its position along the rail relative to the workpiece.
The telescoping inner covers may be in the form of bellows which can extend or contract to accommodate axial movement of the workrest along the elongate rail.
The rigid covers are conveniently in the form of spacers and may be tubular so as to wholly encompass the elongate member, or C-shaped in cross-section to permit their insertion over and removal from the elongate rail as required.
Typically a plurality of cover spacers are provided which fit between, the workrest and appropriate. surfaces extending perpendicularly to the elongate workrest support rail, so that when fitted between the said surfaces and the workrest, the latter is held rigidly and fixedly at a single axial position along the support rail and therefore in axial fixed relationship to the workpiece axis, so that the workrest will always align with a similar region of each workpiece which is mounted between the headstock and tailstock centres on the said workpiece axis.
Typically the alignment is such as to correspond with a cylindrical region of the workpiece near the mid-position of the length of the workpiece measured between the two centres.
The invention is not limited to a single workrest but envisages the mounting of two or more workrests along the said elongate support rail for positioning against other cylindrical regions of a composite workpiece as aforesaid, such that as the said other cylindrical regions are ground they can be engaged by a workrest to resist sideways deformation of the workpiece as the grinding wheel is forced against diametrically opposite regions of the workpiece to grind the particular regions thereof.
Where a composite workpiece includes for example three spaced apart cylindrical regions which are to form the inner surfaces of journal bearings, three workrests are typically provided and in this event four rigid covers are provided each of an axial extent sufficient to just space the two outer workrests accurately relative to the central workrest, and the two outer workrests from fixed end faces at opposite ends of the elongate support rail on which the workrests slide.
Alternatively three such rigid covers may be provided for spacing the first of the workrests accurately relative to the headstock end of the workpiece, the second workrest relative to the said first workrest, and the third workrest relative to the second middle workrest, and clamping means is provided to retain the third workrest axially in position along the elongate rail and to maintain the assembly of spacing covers and workrests between a face of the headstock (or a fixture at the headstock end of the elongate rail), and the said clamping means.
In addition or alternatively to the spacing achieved by means of the said rigid covers, the workrests may include clamping means, grub screws, wing nuts or other devices for securing each workrest at a desired position along the length of the elongate rail. However the spacing achieved by individual accurately machined spacers each forming a cover for providing at least partial protection for the elongate rail, may be preferred, since this does not involve the need for individual clamping or tightening mechanisms which could damage the surface of the rail.
Preferably axial force is applied to the horizontal stack of workrest(s) and spacers by means of a thrust member acting through the tailstock so as to clamp the stack against a face of the headstock assembly, or a structure associated with or forming part of the headstock assembly.
In a method of setting up a grinding machine as aforesaid, the workpiece may be replaced by a setting up bar having cylindrical (journal bearing) regions machined therealong corresponding to diameter and axial extent and axial positions to the cylindrical (journal bearing) region of the workpiece to be ground, and the workrests are positioned both axially and radially so as to be appropriately positioned for engaging similar workpieces as they are loaded successively onto the machine.
The thrust member may be in two parts separable by an actuator, one part acting on the end of the stack through the tailstock, and the other engaging a fixed structure such as a dressing wheelhead assembly mounted on the machine bed. Spring means may be incorporated in the thrust member.
Each workrest preferably comprises a housing which is slidable along the elongate support rail and clamped in position axially, and jaws which can be advanced and retracted relative to the housing to engage a region of a workpiece. The jaws may be driven in and out by electrical or pneumatic or hydraulic drive means.
Where the workpiece is to be in axial compression independently of the axial compressive forces acting on the stack, a subsidiary housing containing the tailstock centre is slidable and adjustably mounted on the main tailstock showing which engage the said stack.
The invention is not limited to the grinding of one type of composite workpiece but can be applied to a grinding machine which under CNC control can move the wheelhead so as to either follow the eccentric throw of cam lobes of a camshaft, or the circular rotation of crankpins about the central axis of a crankshaft, so as to permit the grinding of the journal bearing regions as well as the cam lobes or crankpins of camshafts and crankshafts.
In accordance with another aspect of the invention, in a method of grinding a composite workpiece on a grinding machine as aforesaid, cylindrical journal bearing regions of the workpiece are first of all ground and after at least the first said cylindrical region has been finish ground, a workrest is engaged therewith, positioned as appropriate along the length of the workpiece axis, and after the cylindrical regions of the composite workpiece have all been ground, the wheelhead control is altered, and each of the non-cylindrical regions of the workpiece are ground in turn, the workrest remaining in position against the first to be ground of the cylindrical regions of the workpiece during all of the subsequent grinding operations of the workpiece.
Where the required stiffness can only be achieved by the use of two or more workrests, an appropriate number of such workrests are provided, and the control system is arranged to move each of the workrests into engagement with cylindrical regions of the workpiece after each said region is finish ground.
Since the first cylindrical region of the workpiece has to be ground without a workrest to resist the grinding forces, the first grinding step is preferably performed at a lower material removal rate and with reduced speed of advance of the grinding wheel so as to reduce the grinding forces exerted on the workpiece while the unsupported cylindrical region is ground. According to a preferred feature of the method, after an initial grind of the first cylindrical region, the workrest is introduced against that region which is still to be finished and the workrest is kept in position for the remainder of the grinding of the first said region.
Other workrests can be introduced and engaged against other cylindrical regions as they are ground in a similar way.
Once the workpiece has become supported by at least one workrest, grinding speeds and material removal rates can be increased within the limitations of the machine and grinding medium, so that the overall grinding time of the composite workpiece can be optimised. In particular the grinding of the non-circular or off-axis regions of cam lobes of camshafts and crankpins of crankshafts can be significantly increased in view of the presence of the workrests, so that the finish grinding of the non-circular and off-axis regions of a composite workpiece can be very much quicker than would otherwise be the case. This advantage, coupled with the saving in time which is realised by not having to demount a workpiece and remount it on a new machine, means that the overall machining time for a composite workpiece is significantly reduced.
Where the workpieces are either hollow or have hollowed ends, the headstock and tailstock centres may be in the form of conical workpiece-engaging devices, and where drive is to be transmitted to the workpiece this can be effected either by means of a positive link such as a key, or chuck, or peg drive where a peg enters an off-axis hole provided in the end surface of the workpiece.
However according to a preferred feature of the invention, where the workpiece does not have sufficient material in the end face which is to be driven to provide notches or apertures or openings for receiving pegs or other such driving devices a method of driving the workpiece involves providing an axial compressive force between headstock, workpiece and tailstock, and providing a good friction fit between a driven centre typically at the headstock and the hollow end of the workpiece engaged hereon. Drive can be transmitted to the workpiece with sufficient precision and lack of slip as to allow not only the cylindrical workpiece regions to be ground but also the circular and even off-axis regions to be ground, where the driving torque required to maintain rotation of the workpiece particularly during high metal removal rates, can be quite considerable.
According to a preferred aspect of this last feature of the invention the surface of the driving cone is preferably impregnated with diamond grit so as to provide a very hard but precision surface for engaging in a central circular opening in the end of a composite workpiece as aforesaid, and the axial compressive force exerted between the workpiece and the centres at the opposite ends of the workpiece, is sufficient to cause the grit to bite into the end surface of the workpiece material and accurately centre, and resist any relative movement between, the workpiece and the conical driving cone.
Since accurate circular indexing of a composite workpiece is needed to allow the CNC control of the wheelhead to enable non-circular and off-axis regions to be accurately ground, the workpiece must either incorporate an index mark which can be detected by means of a suitable optical or other sensing device associated with the grinding machine, to provide an index signal to the programmable computer based control system, or a vee notch indexing device may be provided under the control of the computer based control system which after the cylindrical regions have been ground, is advanced so as to engage around one of the non-circular or off-axis components, and after centering the component relative to the notch, provides a zero position for a rotatable indexing device associated with the workpiece drive, to allow accurate indexing of the workpiece thereafter under computer control, to present first one and then another of the non-circular or off-axis regions for grinding.
Since the different cylindrical and non-circular or off-axis regions are located at different axial positions along the length of the workpiece, means is provided for indexing the wheelhead relative to the workpiece or the workpiece relative to the wheelhead so as to enable the grinding wheel to address different regions of the workpiece as required.
Whilst the arrangement of cover means or spacers serves to accurately and firmly locate the workrests relative to the workpiece, there may occur a lack of rigidity in a plane perpendicular to the elongate rails, i.e., in the form of sway of the workrests. Also crabbing movement along the rails can result in misalignment of the workrests relative to the axis of rotation of the workpiece.
According to a further aspect of the present invention, such undesirable movement of the workrests relative to the rail can be obviated by providing for a clamping engagement between the workrest and one of the rails, the clamping being effected after the workrest has been slid into position along the rails, to retain the workrest in that position during the machining operation.
Where one of the two rails underlies the,jaws at the front of the workrest and the other rail, underlies the rear of the workrest, the clamp preferably engages the rail.
Preferably a first bearing assembly is provided below the front of the workrest for running on the front rail.
Undesirable crabbing of the carriage relative to the front rail can be avoided by incorporating roller bearings in the said first bearing assembly.
Preferably the cross-section of each of the rails is in the form of an I-beam, and the first bearing assembly located below the workpiece engaging jaws is adapted to engage opposite sides of the rail.
Preferably the shoulders of the upper and lower enlarged regions of the rail taper towards the narrow stem of the rail and the first bearing assembly includes rolling elements which are set at angles to complement the trapezoidal shape on each side of the rail.
In the case of the rear rail, a second bearing arrangement is provided which is adapted to engage one side of the rear rail and a moveable member is adjustably secured to the carriage for engaging the opposite side of the rear rail, and means is provided for tightening the movable member against the rear rail to clamp the rail between the fixed and movable members.
Preferably a clearance is provided between the rail which is to be clamped and the various surfaces of the second bearing arrangement to permit free travel when the clamping member has been disengaged from the rail and is in its unclamped condition. The sliding movement of the workrest is therefore governed by the engagement between the first bearing assembly and the front rail, and preferably a low friction engagement is ensured with lubrication as appropriate.
Preferably pre-loaded rolling bearings providing high stiffness are utilised.
The bearings may be double-sealed and/or axial sealed.
Conveniently the bearings on the rails are double wiped.
A preferred bearing assembly comprises the IKO LRXDC35.
A preferred rail comprises an IKO linear stainless steel rail.
Preferably a bearing seal assembly is provided around each bearing assembly and rail associated therewith, to prevent the ingress of dirt and/or mechanical particles.
The invention is not limited to the use of a single grinding wheel, but may be adapted to multiple grinding wheels enabling two or more regions of the workpiece to be ground simultaneously.